This invention relates to an improved process for the production of needle coke and to the highly crystalline coke product thereby produced. The highly crystalline needle coke product of this invention possesses remarkably superior properties, especially suitable for the production of graphite electrodes.
The grades of coke generally obtainable by thermal processing of coal or heavy petroleum stocks are not suitable for manufacture of graphite electrodes because of their noncrystalline nature and their contamination with metals. There has developed technology for the production from petroleum stocks of needle coke having a suitably crystalline character and capable of thermal conversion into a highly graphitic coke. Such highly crystalline graphitic cokes command a premium market for use in high-power electrodes.
Processes for the production of suitable needle coke require that pitch and similar precursors of noncrystalline coke be removed from the petroleum stock which may, desirably, include residues from catalytic cracking, thermal cracking, and crude distillation processes. Unstable components that tend to form pitch are removed by a combination of heat-treating and distillation steps. The remaining heavy petroleum oil may then be coked thermally, as by delayed coking, to yield an improved crystalline coke, or needle coke. Another route to an improved high-crystalline coke requires careful segregation of petroleum residual stocks lacking in those precursors which lead to amorphous or metals-contaminated coke.
The use of sulfur, in elemental or mercaptan form, to eliminate sludging and caking during the removal of pitch-forming components from a petroleum oil, is described in U.S. Pat. No. 3,687,840. The use of added sulfur as a means for developing a pitch-free coking feedstock has been carried further in U.S. Pat. No. 4,108,798.
Cokes suitable for the production of highly graphitic electrodes are generally characterized as having a low coefficient of thermal expansion, no greater than about 1.0.times.10.sup.-6 /.degree.C. when measured in the direction of extrusion (with the grain) over the range from 100.degree. to 400.degree. C. Similarly, suitable cokes possess a maximum transverse magnetoresistance of at least about 16.0%.